Noncoherent uwb cooperative communications system

ABSTRACT

This invention relates to a cooperation procedure between terminals in a UWB pulse telecommunications system. A source terminal transmits a data symbol for a recipient terminal in the form of a signal, called a source signal, constituted by a sequence of identical frames, each frame containing a pulse that is position-modulated by means of a PPM modulation alphabet. A relay terminal, receiving said source signal, detects a modulation position of a pulse in at least one frame of the signal received and transmits a relayed signal containing, in at least one predetermined subsequent frame of this signal, a pulse the position of which is obtained on the basis of the first position by a permutation of the PPM modulation alphabet.

TECHNICAL FIELD

This invention relates to the fields of ultra wide band (or UWB)telecommunications and cooperative telecommunications systems.

PRIOR ART

UWB telecommunications systems have been the subject of considerableresearch in recent years. These systems have the special feature ofworking directly in baseband on so-called ultra wide band signals. ByUWB signal, we generally mean a signal conforming to the spectral maskstipulated in the FCC regulations of 14 Feb. 2002, revised in March2005, i.e., basically a signal in the spectral band 3.1 to 10.6 GHzhaving a bandwidth of at least 500 MHz to −10 dB.

UWB signals can be divided into two categories: multi-band OFDM(MB-OFDM) signals and UWB pulse signals. A UWB pulse signal isconstituted by very short pulses, on the order of a few hundredpicoseconds to a nanosecond. The description below will be limited toUWB pulse systems.

UWB systems are suitable for wireless personal networks (WPAN). In aconventional wireless network, such as a cellular telecommunicationsnetwork, the connections are established between a transmitter and areceiver, without the participation of third-party terminals. To improvethe spatial coverage of the wireless networks, ad-hoc architecturesimplementing strategies for cooperation between terminals have beenproposed. FIG. 1 diagrammatically shows cooperation strategy in such anetwork. It is assumed that a source terminal s transmits a data flow toa recipient terminal d. The terminal r also receives the data flow sentby s and relays it to the recipient terminal d. The terminal r thuscooperates in the transmission of data between terminals s and d. Forexample, if the s-d channel is of poor quality, in particular due to thepresence of an obstacle between s and d (NLOS configuration), the s-r-dchannel enables it to be bypassed and allows a connection ofsatisfactory quality to be obtained. The data flow can be relayed by aplurality of terminals in order to further increase the spatialdiversity of the transmission paths. In addition, it can be relayed in aone hop (single-hop) or in a plurality of successive hops(multiple-hop).

Families sharing access in wireless networks are known: TDMA (TimeDivision Multiple Access), CDMA (Code Division Multiple Access), SDMA(Space Division Multiple Access). In a TDMA network, each terminal has atransmission interval dedicated to it. Two possible modes of cooperationin a cooperative TDMA network are distinguished: parallel cooperationand serial cooperation.

In a parallel cooperation mode, the relay terminal receives the datafrom the source terminal during the transmission interval allocated tothe latter and retransmits it to the recipient terminal during itsspecific transmission interval. The recipient terminal thus receives thesame data, via different paths, first during the transmission intervalof the source terminal and second during the transmission interval ofthe relay terminal. Although the term “parallel” may appear to be poorlychosen due to the sequential reception of data by the recipientterminal, it in fact refers to the absence of interference between thetwo paths, resulting from the time separation of the transmissionintervals of the source and relay terminals. The operation in parallelcooperation mode assumes that the relay terminal does not have specificdata to be transmitted during its transmission interval. This limitationsubstantially reduces the possibilities of cooperation.

In a serial cooperation mode, the relay terminal receives data from thesource terminal during the transmission interval of the latter andretransmits it in the same interval. The relay terminal transmits onlyits specific data during its transmission interval. The recipientterminal thus receives the data from the source terminal, via twodifferent paths during a transmission interval of the source terminal.

Due to the simultaneous transmission of specific data and relayed dataduring the same transmission interval, this data must be examined inorder to ensure its orthogonality at the reception. This code isreferred to as a distributed space-time code or DSTC.

Cooperative telecommunications systems are, like so-called MIMO(Multiple-Input Multiple-Output) systems, systems with transmissionspatial diversity. Cooperative telecommunications systems are also onelegant way of emulating a MIMO system from single-antenna terminals.The type of detection used in a MIMO system or a cooperative systemreceiver depends on the data available on the channel. The following canbe distinguished:

so-called coherent systems, in which the receiver knows thecharacteristics of the transmission channel, typically owing to achannel estimation performed using pilot symbols transmitted by thetransmitter. The channel estimation is then used to detect data symbols.Coherent systems are generally intended for high rate applications;

non-coherent systems, in which the receiver performs a blind detectionof the data symbols, without prior knowledge of the transmission channelcharacteristics;

differential systems, in which the data symbols are coded in the form ofphase or amplitude differences between two consecutive transmissionsymbols. These systems may not require knowledge of the channel on thereceiver side.

An example of a coherent cooperative system is known from the article ofS. Yang and J.-C. Belfiore, entitled “Optimal space-time codes for theMIMO amplify-and-forward cooperative channel” available at the websitewww.comelec.enst.fr. An example of a differential cooperative system wasproposed in the article of V. Tarokh et al., published in IEEE Journalon selected areas in communications, Vol. 18, No. 7, July 2000.

While the systems in the aforementioned articles work well fornarrowband signals, this does not apply to UWB signals. Indeed, thesesystems use codes with complex coefficients. However, in considerationof the very short duration of the pulses used, it is not possible torecover phase data in a UWB signal.

A coherent cooperative system using UWB signals and a code with realelements was proposed in the article of C. Abou-Rjeily et al., entitled“Distributed algebraic space time codes for ultra-widebandcommunications” submitted for publication in Kluwer Journal (Springerspecial issue on Cooperation in Wireless Networks). However, asmentioned above, such a system requires the receiver to have knowledgeof the transmission channel.

The objective of this invention is to propose a robust non-coherentcooperative UWB system with a particularly simple architecture.

DESCRIPTION OF THE INVENTION

This invention is defined by a cooperation procedure between terminalsin a UWB pulse telecommunications system in which a source terminaltransmits a data symbol for a recipient terminal in the form of asignal, called a source signal, constituted by a sequence of identicalframes, said sequence corresponding to a symbol time, with each framecontaining a pulse that is position-modulated by means of a PPMmodulation alphabet. A relay terminal, receiving said source signal,detects a modulation position of a pulse in at least one frame of thesignal received, called the first position, and transmits a relayedsignal containing, in at least one predetermined subsequent frame ofsaid symbol time, a pulse of which the position, called the secondposition, is obtained on the basis of the first position by apermutation of the PPM modulation alphabet.

The invention also relates to a transmission/reception module of a UWBpulse telecommunications terminal intended to serve as a relay terminal,including:

means for time slicing of the signal received according to a pluralityof consecutive time windows corresponding to the frames of a UWB signaltransmitted by a source terminal, thus obtaining a plurality of framesignals;

means for measuring, for at least one frame, the energy of the framesignal in a plurality of modulation positions of a predetermined PPMalphabet;

means for detecting the position corresponding to the highest of theenergies thus measured in said frame;

means for permutation of the positions of said PPM alphabet in order toobtain a permuted position from the position thus detected;

means for generating a pulse in said permuted position during a frame ofthe signal received following said frame.

The invention finally relates to a reception module of a UWB pulsetelecommunications terminal in which the data symbol is transmitted inthe form of a signal, called a source signal, constituted by a sequenceof identical frames, with each frame containing a pulse that isposition-modulated by means of a PPM modulation alphabet, whichreception module includes:

means for time slicing of the time signal according to a plurality oftime windows each corresponding to two consecutive frames of the sourcesignal, called first and second frames;

means for measuring the energy of the signal received, in each firstframe, at each PPM alphabet modulation position, thus obtaining a firstenergy value for each modulation position;

means for measuring the energy of the signal received, in each secondframe, at each PPM alphabet modulation position and at positionsobtained from the latter by means of a predetermined permutation, thusobtaining second and third energy values for each modulation position;

means for summing, for each modulation position, the first, second andthird energy values, thus obtaining a fourth energy value for eachmodulation position;

means for summing, for each modulation position, the fourth energyvalues obtained for the different time windows, thus obtaining a fifthenergy value for each modulation position;

means for determining the modulation position corresponding to thehighest of said fifth energy values;

means for determining, on the basis of the position thus obtained, saiddata symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear on readingabout a preferred embodiment of the invention, in reference to theappended figures, in which:

FIG. 1 diagrammatically shows a known cooperation strategy in a wirelessnetwork;

FIG. 2A shows a UWB pulse signal transmitted by a source terminal;

FIG. 2B shows a UWB pulse signal transmitted by a relay terminal;

FIG. 2C shows a UWB pulse signal received by a recipient terminal in acooperative network using the cooperation procedure according to theinvention;

FIG. 3 shows the choice of a relay terminal by joint action between asource terminal and a recipient terminal;

FIG. 4 diagrammatically shows a transmission/reception module of a relayterminal according to an embodiment of the invention;

FIG. 5 diagrammatically shows a reception module of a recipient terminalaccording to an embodiment of the invention;

FIG. 6 shows the detail of a unit of the reception module of FIG. 5.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The basic idea of the invention is to use a cooperation strategy usingUWB signals with pulse position modulation or PPM while ensuring theorthogonality between the signal to be relayed and the signal relayed.

FIG. 2A shows such a UWB signal with PPM modulation. A data symbol to betransmitted is coded on a symbol time T_(s)=N_(f)T_(f) where T_(f) is aframe duration. Each frame is itself divided into a plurality M of basicintervals of duration T_(c) also called pulse repetition intervals orPRI.

The UWB pulse transmitted by the source terminal k can be expressed by:

$\begin{matrix}{{s_{k}(t)} = {A_{k}^{s}{\sum\limits_{n = 0}^{N_{f} - 1}{p\left( {t - {n\; T_{f}} - {d_{k}T_{c}}} \right)}}}} & (1)\end{matrix}$

where p(t) is the waveform of the basic pulse, A_(s) ^(k) is acoefficient dependent on the transmission power and d_(k)ε{0, . . . ,M−1} is the M-ary PPM position of the symbol to be transmitted. Mrepresents the cardinality of the PPM modulation alphabet. It is notedthat the modulation position d_(k) is identical in the N_(f) differentframes constituting the signal. Indeed, it is not necessary to providedistinct time-hopping sequences for the different users as in TH-UWB(Time Hopping UWB), as the access separation is produced by distincttransmission intervals. Each transmission time interval or TTI isdivided into a plurality of symbol periods T_(s).

The duration T_(c) of the basic interval is advantageously chosen so asto be greater than the spread of the channel, generally on the order of100 ns. A separation of pulses upon reception is thus obtained withoutrequiring a preliminary equalisation.

The relay terminal r receives the signal transmitted by the sourceterminal s during the transmission time terminal (TTI) allocatedthereto. The signal received by the relay terminal can be expressed by:

$\begin{matrix}{{\rho_{k}(t)} = {{A_{k}^{s}{\sum\limits_{n = 0}^{N_{f} - 1}{{p\left( {t - {n\; T_{f}} - {d_{k}T_{c}}} \right)} \otimes {h_{sr}(t)}}}} + {w(t)}}} & (2)\end{matrix}$

where h_(sr) is the impulse response of the channel between s and r,assumed to have a time support lower than T_(c), and w is a random noisefunction.

The relay terminal performs a non-coherent detection in the even framesn (the first frame of the symbol being indexed by 0). To do this, therelay terminal calculates on-the-fly the energy of the signal receivedfor the various modulation positions in the current even frame, namely:

$\begin{matrix}{{ɛ_{k}^{n}(m)} = {\int_{{nT}_{f} + {mT}_{c}}^{{nT}_{f} + {{({m + 1})}T_{c}}}{{{\rho_{k}(t)}}^{2}{t}}}} & (3)\end{matrix}$

and determines, for each even frame n, the position corresponding to thesymbol transmitted by means of:

$\begin{matrix}{{\hat{d}}_{k}^{n} = {\underset{m}{Arg}\mspace{11mu} {\max \left( {ɛ_{k}^{n}(m)} \right)}}} & (4)\end{matrix}$

It is noted that if the detection is correct, we of course have:

{circumflex over (d)} _(k) ^(n) ==d _(k), even ∀n, 0≦n≦N _(f)−1  (5)

Based on the value of the position detected in frame n, the relayterminal transmits, to the next frame n+1, a relayed signal. The relayedsignal can be expressed by:

$\begin{matrix}{{s_{k}^{r}(t)} = {A_{k}^{r}{\sum\limits_{\underset{n\mspace{14mu} {pair}}{n = 0}}^{N_{f} - 1}{p\left( {t - {\left( {n + 1} \right)T_{f}} - {{\sigma \left( {\hat{d}}_{k}^{n} \right)}T_{c}}} \right)}}}} & (6)\end{matrix}$

where A_(k) ^(r) is a coefficient dependent on the transmission power ofthe relay terminal and where σ is a permutation of the alphabet {0, . .. , M−1}, for example a circular permutation. A permutation is anybijection of {0, . . . , M−1} on itself, except the identity. A circularpermutation σ is defined by the relation σ(m)=m+q(mod M) where q is aninteger such that 0<q≦M−1.

The signal relayed in FIG. 2B is shown, with the assumption that thedetection by the relay terminal was correct in the sense of (5). It isnoted that the relayed signal is present only in the odd frames (n=1;n=3) and that the PPM position inside these frames has been subjected toa permutation with respect to those of the source signal, here acyclical rotation by one position to the right, i.e. σ(m)=m+1(mod M).

The signal received by the recipient terminal d can be expressed by:

$\begin{matrix}{{R_{k}(t)} = {{A_{k}^{s}\; {\sum\limits_{n = 0}^{N_{f} - 1}{{p\left( {t - {nT}_{f} - {d_{k}T_{c}}} \right)} \otimes {h_{sd}(t)}}}} + {A_{k}^{r}{\sum\limits_{\underset{n\mspace{14mu} {pair}}{n = 0}}^{N_{f} - 1}{{p\left( {t - {\left( {n + 1} \right)T_{f}} - {{\sigma \left( {\hat{d}}_{k}^{n} \right)}T_{c}}} \right)} \otimes {h_{rd}(t)}}}} + {v(t)}}} & (7)\end{matrix}$

where h_(rd)(t) is the impulse response of the channel between r and d,with a time support lower than T_(c) and where v(t) is a random functionrepresenting the noise. It should be noted in (7) that the principle ofcooperation between the terminals is particularly robust since theorthogonality between the direct and relayed signals is ensured by asimple predetermined time permutation on the PPM positions. Indeed, ifthe detection by the relay is correct: σ({circumflex over (d)}_(k)^(n))=σ(d_(k))≠d_(k), and it is noted that there is no overlapping ofpulses because T_(c) is greater than the spread of the channel (s-r ors-r-d).

The recipient terminal performs a non-coherent detection on the signalreceived. To do this, it calculates the total energy, for each PPMposition, namely:

$\begin{matrix}{{E_{k}(m)} = {{\sum\limits_{n = 0}^{N_{f} - 1}\left( {\int_{{nT}_{f} + {mT}_{c}}^{{nT}_{f} + {{({m + 1})}T_{c}}}{{{R_{k}(t)}}^{2}{t}}} \right)} + {\sum\limits_{\underset{n\mspace{14mu} {pair}}{n = 0}}^{N_{f} - 1}\left( {\int_{{nT}_{f} + {{\sigma {(m)}}T_{c}}}^{{nT}_{f} + {{({{\sigma {(m)}} + 1})}T_{c}}}{{{R_{k}(t)}}^{2}{t}}} \right)}}} & (8)\end{matrix}$

The first term of (8) corresponds to the signal received directly fromthe source terminal, while the second term corresponds to the signalreceived via the relay terminal. The M-ary symbol detected is thenobtained from the decision:

$\begin{matrix}{{\hat{d}}_{k} = {\underset{m}{Arg}\mspace{11mu} {\max \left( {E_{k}(m)} \right)}}} & (9)\end{matrix}$

It is noted that neither the recipient terminal nor the relay terminalneeds to carry out a channel estimation.

The choice of coefficients A_(k) ^(s), A_(k) ^(r) and therefore oftransmission powers of the source and relay terminals is dependent onthe operating conditions.

We will first consider the case in which the respective conditions ofthe s-d and r-d transmission channels are not known. The coefficients inquestion can be chosen according to two distinct modes:

according to a first mode, the powers transmitted by the source terminaland the relay terminal are chosen so that their sum complies with theaforementioned FCC spectral mask. In other words, if P is the value ofthe power enabling compliance with the spectral mask, the powers of thesource terminal and the relay terminal will be chosen to be equal to2P/3 and P/3, respectively. Given that the relay terminal sends half thenumber of pulses as the source terminal, it in fact amounts to choosing

${A_{k}^{s}}^{2} = {{A_{k}^{r}}^{2} = {\frac{1}{2}A^{2}}}$

where A is the value of the amplitude that would have enabled thespectral mask to be satisfied with the source terminal alone. It is thusunderstood that the first mode can enable, for the same BER, the powerof the source terminal to be saved by distributing it between the sourceand relay;

according to a second mode, the respective powers of the source andrelay terminals also individually satisfy the spectral mask. In thiscase, the total power transmitted is twice the one that would have beentransmitted by the source terminal alone. We then have

${A_{k}^{s}}^{2} = {{\frac{1}{2}{A_{k}^{r}}^{2}} = {A^{2}.}}$

The second mode makes it possible to obtain a 3-dB increase in power atthe reception. In other words, with respect to the first mode above, oran operation without a relay, it is possible to obtain the same BER fora signal-to-noise ratio half that of the first mode of operation or anoperation without a relay.

If the conditions of the s-d and r-d channels are known, for example theattenuation coefficients on these channels, the distribution of powerbetween the source and relay terminals according to the first mode is nolonger done by taking into account only the number of pulses transmittedin s_(k) and s_(k) ^(r), but it also involves taking into account theattenuation conditions. The transmission powers of s and r are thenchosen so that:

P _(s) =aP and P _(r) =bP with a+b=1  (10)

where the weighting coefficients a and b are determined, for example, onthe basis of the attenuation coefficients of the s-d and r-d channels.

The coefficients a and b can alternatively be determined on the basis oftransmission power control loops. To do this, power control indications,TPC_(s) and TPC_(d) (Transmission Power Control) are sent by theterminal d, via two return paths, to the terminals s and r. This assumesthat a separate detection of the direct signal and of the relayed signalis periodically performed. According to the indications TPI_(s) andTPI_(d), the terminal s decrements/increments a and the terminal rincrements/decrements b so that the sum of the weighting coefficientsa+b remains equal to 1.

According to an alternative corresponding to an operation according tothe second mode, it is possible to have independent return paths,wherein the coefficients a and b are no longer linked, but each remainslower than 1 so as to comply with the spectral mask.

In the cooperation strategy described above, it is assumed that there isa given relay terminal r. However, as a general rule, a plurality ofterminals can perform the relay function. It is then necessary to make aselection.

Advantageously, according to a first alternative embodiment, the choiceof the relay terminal is made by the cooperation between the sourceterminal s and the recipient terminal d on the basis of a proximitycriterion. It is first assumed that the terminals can determine thedistances separating them (peer-to-peer ranging) according toconventional pseudo-distance or round-trip propagation time calculationmeans. The UWB signals are suitable, due to their waveforms (short timepulses), for a location application. For example, there is a descriptionof a method for calculating distances between UWB terminals in thearticle of Neiyer S. Correal et al., entitled “An UWB relative locationsystem” available at the website www.ee.vt.edu.

The terminals s and d first determine the distance D_(s-d) thatseparates them. The terminal s then determines the set S_(s) of itsclose neighbours: to do this, it measures the distances that separate itfrom the surrounding terminals and selects those located less thanD_(s-d) from it. The terminal d similarly determines the set S_(d) ofits close neighbours. The relay terminal is selected in the setS_(s)∩S_(d) as the terminal minimising the sum D_(s-r)+D_(r-d) whereD_(s-r) and D_(r-d) are the distances between s and r and between r andd. If the set S_(s)∩S_(d) is empty, the cooperation procedure istemporarily abandoned.

According to a second alternative embodiment, the relay terminal isselected in the set S_(s)∩S_(d), on the basis of an error rate (BER). Todo this, the source terminal transmits a sequence of predeterminedsymbols, called a preset sequence, for example a control channelsequence or a pilot symbol sequence, to the surrounding terminals. Thissequence is known to all of the terminals, and each terminal thatreceives it can thus determine its BER by a comparison between thedetected sequence and the preset sequence. Those belonging toS_(s)∩S_(d) and of which the BER is lower than a threshold value thensend a message of acknowledgement to the source terminal, optionallyspecifying the range of error rate measured and/or the current load ofthe terminal. The source terminal selects the relay terminal from them.

FIG. 4 diagrammatically shows an embodiment of thetransmission/reception module of a relay terminal according to theinvention.

The signal of the source terminal is received via the duplexer 410. Theduplexer 410 is in the reception position for the even frames and in thetransmission position for odd frames. The signal received undergoes atime slicing for each PPM alphabet modulation position in the module420. The time slicing of 420 is triggered at the beginning of each evenframe and lasts for a time T_(c) for each of the M modulation positions.The quadratic integrators 425 ₀, . . . , 425 _(M-1) respectively receivethe signals thus obtained and measure the energy values ε_(k) ^(n)(m),m=0, . . . , M−1. These values are provided in analogue form to thedecision module 430. Said module performs a comparison and detects themost energetic position. It provides, at the output, a word of M bitscoding the value {circumflex over (d)}_(k) ^(n), for example by means ofa bit equal to 1 for the modulation position detected and to 0 for theother bits.

This word then undergoes a circular permutation σ of its bits, in thiscase performed by wiring, before being transformed into a sequence of Mbits by the parallel-serial converter 440. The bit sequence is thenmodulated by a pulse modulator 450. More specifically, this modulatortransforms a sequence c₀, . . . , c_(M-1) of M bits into an analoguesignal

$\sum\limits_{m = 0}^{M - 1}{c_{m} \cdot {{p\left( {t - {mT}_{c}} \right)}.}}$

This signal then undergoes a delay of T_(f)−τ in the delay line 460where τ is the processing duration in the stages 410 to 450 before beingtransmitted, via the amplifier 480 and the duplexer 410. Reference 470represents a switch upstream of the amplifier 480, receiving at itsinputs the relayed signal s_(k) ^(r)(t) and the own signal s_(k′)(t) ofthe relay terminal. The switch is in position 471 during an interval TTIallocated to the source terminal and in position 473 during an intervalTTI allocated to the relay terminal.

FIG. 5 diagrammatically shows an embodiment of the reception module ofthe recipient terminal according to the invention.

It is assumed in this figure that there is an even number 2n_(f) offrames, i.e. N_(f)=2n_(f)−1. The signal received R_(k)(t) is delayed bythe (n_(f)−1) delay lines 510, each with a delay value 2T_(f).

The signal R_(k)(t) and the delayed signals are processed by n_(f)modules 520 ₀, . . . , 520 _(n) _(f) ₋₁, with each of these modulesperforming a processing operation on two consecutive frames of thesignal received.

FIG. 6 diagrammatically shows the structure of a module 520 _(i). Thesignal at the input is provided to a first time slicing module 620 ₀and, to a second time slicing module 620 ₁ through a delay line 610 ofvalue T_(f). The modules 620 ₀ and 620 ₁ have identical structures andperform a slicing for each modulation position, like the modules 420described above, with module 620 ₀ taking charge of the even frame 2iand module 620 ₁ taking charge of the odd frame 2i+1. The energies ofthe signals thus sliced are obtained by means of 2M quadraticintegrators referenced 625 ₀, . . . , 625 _(M-1) for the even frame and626 ₀, . . . , 626 _(M-1) for the odd frame. These energies are summedby means of summers 630 ₀, . . . , 630 _(M-1), with each summer 630 _(m)being connected at the input to integrator 625 _(m) and to the twointegrators 626 _(m) and 626 _(σ(m)). The summer 625 _(m) thus provides,at the output, the value:

$\begin{matrix}{\left( {\int_{{nT}_{f} + {mT}_{c}}^{{nT}_{f} + {{({m + 1})}T_{c}}}{{{R_{k}(t)}}^{2}\; {t}}} \right) + \left( {\int_{{{({n + 1})}T_{f}} + {mT}_{c}}^{{{({n + 1})}T_{f}} + {{({m + 1})}T_{c}}}{{{R_{k}(t)}}^{2}\; {t}}} \right) + \left( {\int_{{{({n + 1})}T_{f}} + {{\sigma {(m)}}T_{c}}}^{{{({n + 1})}T_{f}} + {{({{\sigma {(m)}} + 1})}T_{c}}}{{{R_{k}(t)}}^{2}\; {t}}} \right)} & (11)\end{matrix}$

where n=2i is the (even) rank of the frame taken charge of by module 620₀.

Now turning back to FIG. 5, the n_(f) outputs of the modules 520corresponding to the same modulation position m are summed by means of asummer 530 m. The outputs of these M summers represent the decisionvariables E_(k)(m) of the equation (8). The module 540 performs acomparison of the values E_(k)(m), m=0, . . . , M−1 and determines themost energetic position {circumflex over (d)}_(k), in accordance with(9). In 550, it deduces the data symbol corresponding to said modulationposition.

In the above description of the invention, it was assumed that the relayterminal had a “relaying rate of ½”, with the even frames of the signalreceived being relayed during the odd frames of this signal. In general,it is understood that the relay terminal can have a “relaying rate” of1/n with n≧2, each nth of the signal received being relayed during asubsequent frame of said signal.

1. Cooperation procedure between terminals in a UWB pulsetelecommunications system, characterised in that a source terminaltransmits a data symbol for a recipient terminal in the form of asignal, called a source signal, constituted by a sequence of identicalframes, said sequence corresponding to a symbol time, with each framecontaining a pulse that is position-modulated by means of a PPMmodulation alphabet, and in that a relay terminal, receiving said sourcesignal, detects a modulation position of a pulse in at least one frameof the signal received, called the first position, and transmits arelayed signal containing, in at least one predetermined subsequentframe of said symbol time, a pulse of which the position, called thesecond position, is obtained from the first position by a permutation ofthe PPM modulation alphabet.
 2. Cooperation procedure according to claim1, characterised in that the second position is obtained by comparingthe energies received in the different positions of the PPM modulationalphabet in said received signal frame.
 3. Cooperation procedureaccording to claim 1 or 2, characterised in that said subsequent frameof the relayed signal is consecutive to said received signal frame. 4.Cooperation procedure according to one of the previous claims,characterised in that said permutation is a circular permutation. 5.Cooperation procedure according to one of the previous claims,characterised in that the transmission powers of the source terminal andthe relay terminal are chosen so as to be equal to a.P and b.P where Pis a power value complying with the UWB spectral mask and where a and bare coefficients such that 0<a<1 and 0<b<1 with a+b=1.
 6. Cooperationprocedure according to claim 5, characterised in that the coefficients aand b are determined according to the respective conditions of thechannel between the source terminal and the recipient terminal and thechannel between the relay terminal and the recipient terminal. 7.Cooperation procedure according to claim 5, characterised in that thecoefficients a and b are controlled by means of power control loops bytwo return paths from the recipient terminal to the source terminal andthe relay terminal, respectively.
 8. Cooperation procedure according toone of claims 1 to 4, characterised in that the transmission powers ofthe source terminal and the relay terminal are each chosen so as to beequal to a power value complying with the UWB spectral mask. 9.Cooperation procedure according to one of the previous claims,characterised in that said relay terminal is determined by the sourceand recipient terminals by means of the following steps: determining thedistance separating the source terminal and the relay terminal;determining a first set of terminals located at a shorter distance thansaid distance from the source terminal; determining a second set ofterminals located at a shorter distance than said distance from thesource terminal; selecting a relay terminal among the terminals commonto said first and second sets, called candidate terminals, as the oneminimising the sum of the distances between the source terminal and thecandidate terminal, and between the candidate terminal and the recipientterminal.
 10. Cooperation procedure according to one of claims 1 to 8,characterised in that said relay terminal is determined by the sourceand recipient terminals by means of the following steps: determining thedistance separating the source terminal and the relay terminal;determining a first set of terminals located at a shorter distance thansaid distance from the source terminal; selecting the relay terminalfrom the terminals common to said first and second sets, calledcandidate terminals; determining a second set of terminals located at ashorter distance than said distance from the source terminal;determining the terminals common to said first and second sets, calledcandidate terminals, and sending a sequence of predetermined symbols bythe source terminal to said candidate terminals, with the relay terminalbeing selected as the candidate terminal detecting said sequence withthe lowest error rate.
 11. Cooperation procedure according to one of theprevious claims, characterised in that said recipient terminal,receiving both the source signal and the relayed signal: measures, foreach frame of a first subset of frames of the received signal, notcorresponding to a frame of the relayed signal, the energies received inthe different positions of the PPM modulation alphabet, thus obtaining afirst energy value for each modulation position; measures, for eachframe of a second subset of frames of the received signal, eachcorresponding to a frame of the relayed signal, the energies received ineach position of said alphabet and in a position obtained on the basisof the latter by said permutation, thus obtaining a second and a thirdenergy value for each modulation position; sums, for each modulationposition, said first, second and third energy values, thus obtaining atotal energy value for each modulation position; compares the totalenergy values of the different modulation positions and determines theposition corresponding to the highest total energy value; determinessaid data symbol on the basis of the position thus obtained. 12.Transmission/reception module of a UWB pulse telecommunicationsterminal, characterised in that it includes: means for time slicing(420) the signal received according to a plurality of consecutive timewindows corresponding to the frames of a UWB signal transmitted by asource terminal, thus obtaining a plurality of frame signals; means formeasuring (425 ₀, 425 ₁, . . . , 425 _(M-1)), for at least one frame,the energy of the frame signal in a plurality of modulation positions ofa predetermined PPM alphabet; means for detecting (430) the positioncorresponding to the highest of the energies thus measured in saidframe; means for permutation of the positions of said PPM alphabet inorder to obtain a permuted position from the position thus detected;means for generating (440, 450, 460) a pulse in said permuted positionduring a frame of the signal received following said frame. 13.Transmission/reception module according to claim 12, characterised inthat the permutation means are suitable for performing a circularpermutation of said alphabet.
 14. Transmission/reception moduleaccording to claim 12 or 13, characterised in that said frame followingthe signal received is consecutive to said frame.
 15. Reception moduleof a UWB pulse telecommunications terminal in which a data symbol istransmitted in the form of a signal, called a source signal, constitutedby a sequence of identical frames, with each frame containing a pulsethat is position-modulated by means of a PPM modulation alphabet,characterised in that it includes: means for time slicing (520 ₀, . . ., 520 _(n) _(f) ₋₁) the time signal according to a plurality of timewindows each corresponding to two consecutive frames of the sourcesignal, called first and second frames (2i, 2i+1); means for measuringthe energy of the signal received (625 ₀, 625 ₁, . . . , 625 _(M-1), 620₀), in each first frame, at each PPM alphabet modulation position, thusobtaining a first energy value for each modulation position; means formeasuring the energy of the signal received, in each second frame (626₀, 626 ₁, . . . , 626 _(M-1), 620 ₁), at each PPM alphabet modulationposition and at positions obtained from the latter by means of apredetermined permutation, thus obtaining second and third energy valuesfor each modulation position; means for summing (630 ₀, . . . , 630_(M-1)), for each modulation position, the first, second and thirdenergy values, thus obtaining a fourth energy value for each modulationposition; means for summing (530 ₀, . . . , 530 _(M-1)), for eachmodulation position, the fourth energy values obtained for the differenttime windows, thus obtaining a fifth energy value for each modulationposition; means for determining (540) the modulation positioncorresponding to the highest of said fifth energy values; means fordetermining (550), on the basis of the position thus obtained, said datasymbol.
 16. Reception module according to claim 15, characterised inthat said predetermined permutation is a circular permutation of saidPPM alphabet.